I. Field of the Invention
This invention relates to methods of powder formation and thin film deposition from reagents contained in liquid or liquid-like fluid solutions, whereby the fluid solution, near its supercritical point temperature, is released into a region of lower pressure causing a superior, very fine atomization or vaporization of the solution. Gasses are entrained or fed into the dispersed solution and rapidly flow into a flame or plasma torch. The reagents react and form either: 1) powders which are collected; or 2) a coating from the vapor phase onto a substrate positioned in the resulting gases and vapors. Release of the near supercritical point temperature fluid causes dispersion and expansion resulting in a very fine nebulization of the solution, which yields improved powder and film quality, deposition rates and increases the number of possible usable precursors.
II. Background of the Invention
Chemical vapor processing has been used extensively for the production of powders and coatings. Chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) is the term used when coatings onto a substrate are formed. CVD production of coatings is widespread. Many of these coating are only nanometers thick and smooth to less than 5% percent of coating thickness. Reaction and agglomeration of the reacted vapor material in the gas stream forms powders which can be commercially useful. In fact, nanopowders are required in the formation of nanomaterials which have different properties from those of bulk materials. These materials"" properties can be tailored by controlling the cluster size of the nanopowder. Similarly, coatings of less than 50 nm can have properties which are different from thicker films, and the properties change further as the coating thins.
It is desirable to form such powders and coatings at low production and capitalization costs and with simple production processes. However, for many materials there is a very limited selection of available precursors which can be vaporized and used for traditional CVD. Being able to form coatings in the open atmosphere tremendously eases substrate handling and flow through the coating process. In addition to thin films low cost quality thick coatings and bulk materials are also desirable.
Combustion chemical vapor deposition (xe2x80x9cCCVDxe2x80x9d), a recently invented CVD technique, allows for open atmosphere deposition of thin films. The CCVD process offers several advantages over other thin-film technologies, including traditional CVD. The key advantage of CCVD is its ability to deposit films in the open atmosphere without any costly furnace, vacuum, or reaction chamber. As a result, the initial system capitalization requirement can be reduced up to 90% compared to a vacuum based system. Instead of a specialized environment, which is required by other technologies, a combustion flame provides the necessary environment for the deposition of elemental constituents from solution, vapor, or gas sources. The precursors are generally dissolved in a solvent that also acts as the combustible fuel. Depositions can be performed at atmospheric pressure and temperature within an exhaust hood, outdoors, or within a chamber for control of the surrounding gasses or pressure.
Since CCVD generally uses solutions, a significant advantage of this technology is that it allows rapid and simple changes in dopants and stoichiometries which eases deposition of complex films. In contrast to conventional CVD, where the precursor vapor pressure is a concern which dictates expensive high vapor pressure precursors, the CCVD technique generally uses inexpensive, soluble precursors. In addition, precursor vapor pressures do not play a role in CCVD because the dissolution process provides the energy for the creation of the necessary ionic constituents. In general, the precursor materials used for traditional CVD depositions are between 10 and 1000 times more expensive than those which can be used in CCVD processing. By adjusting solution concentrations and constituents, a wide range of stoichiometries can be deposited quickly and easily. Additionally, the CCVD process allows both chemical composition and physical structure of the deposited film to be tailored to the requirements of the specific application.
Unlike CVD, the CCVD process is not confined to an expensive, inflexible, low-pressure reaction chamber. Therefore, the deposition flame, or bank of flames, can be moved across the substrate to easily coat large and/or complex surface areas. Because the CCVD process is not limited to specialized environments, the user can continuously feed materials into the coating area without disruption, thereby permitting batch processing. Moreover, the user can limit deposition to specific areas of a substrate by simply controlling the dwell time of the flame(s) on those areas. Finally, the CCVD technology generally uses halogen free chemical precursors having significantly reduced negative environmental impact compared to conventional CVD, resulting in more benign by-products.
Numerous materials have been deposited via CCVD technology with the combustion of a premixed precursor solution as the sole heat source. This inexpensive and flexible film deposition technique permits broader use of thin film technology. The CCVD process has much of the same flexibility as thermal spraying, yet creates quality, conformal films like those associated with CCVD. Traditional CVD often requires months of effort to successfully deposit a material. With CCVD processing, a desired phase can be deposited in a few days and at a fraction of the cost of traditional CVD.
By providing these coating capabilities inexpensively, the CCVD process can broaden the commercial opportunity for thin films, including use in tribological, thermal protective, wear, space environment protective, optic, electronic, structural and chemical resistant applications. Thus, government and commercial users can benefit from the advantages of thin films over thick films, including their high adhesion to the substrate, controlled microstructure, greater flexibility, reduced raw material consumption and reduced effect on the operating characteristics and/or dimensions of the coated system.
Ichinose, H., Shiwa, Y., and Nagano, M., Synthesis of BaTiO3/LaNiO3 and PbTiO3/LaNiO3 Thin Films by Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1, 10 p. 5903-6 (1994) and Ichinose, H., Shiwa, Y., and Nagano, M., Deposition of LaMO3 (M=Ni, Co, Cr, Al)xe2x80x94Oriented Films by Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1, 10 p. 5907-10 (1994) used CCVD processing, which they termed spray combustion flame technique, by ultrasonically atomizing a precursor containing solution, and then feeding the resulting nebulized solution suspended in argon carrier gas into a propane combustion flame. However, these atomization techniques cannot reach the highly desirable submicron capabilities which are important to obtaining improved coating and powder formation.
U.S. Pat. No. 4,582,731 (the xe2x80x9c""731 patentxe2x80x9d) discloses the use of a supercritical fluid molecular spray for the deposition of films. However, the ""731 patent is for physical vapor deposition (PVD), which differs from the independently recognized field of CVD by having no chemical reagents and normally being operated at high vacuum. Additionally, no flame or plasma torch is used in this method, and only supercritical fluid solutions are considered. Chemical reagents are beneficial because of there physical properties, including higher solubility. The flame and plasma torch enable coatings in the open atmosphere with no additional heat source. The ""731 deposition material, however, does not start from a reagent, and thus will not react at supercritical conditions.
U.S. Pat. No. 4,970,093 (the xe2x80x9c""093 patentxe2x80x9d) discloses the use of supercritical fluids and CVD for the deposition of films. Work related to the ""083 patent is described in B. M. Hybertson, B. N. Hansen, R. M. Barkley and R. E. Sievers,. Supercritical Fluid Transport-Chemical Deposition of Films, Chem. Mater., 4, 1992, p. 749-752 and Hybertson et al and B. N. Hansen, B. M. Hybertson, R. M. Barkley and R. E. Sievers, Deposition of Palladium Films by a Novel Supercritical Fluid Transport-Chemical Deposition Process produce, Mat. Res. Bull., 26, 1991, p. 1127-33. The ""093 patent is for traditional CVD without a flame or plasma torch and does not consider open atmosphere capable techniques such as CCVD, which has the associated advantages discussed above. Additionally, only supercritical fluid solutions are considered; liquid solutions near the supercritical point are not addressed. All of the precursors of the ""093 patent are carried in the supercritical solution which can limit the usable precursors due to reactivity and solubility in supercritical fluids.
B. M. Merkle, R. N. Kniseley, F. A. Smith and I. E. Anderson, Superconducting YBaCuO Particulate produced by Total Consumption Burner Process produce, Mat. Sci. Eng., A124, p.31-38 (1990), J. McHale et al., Preparation of High-Tc Oxide Films via Flaming Solvent Spray, 3. Supercond. 5 (6), p.511 (1992), and M. Koguchi et al., Preparation of YBa2Cu3Ox Thin Film by Flame Pyrolysis, Jpn. J. Appl. Phys. 29 (1), p.L33 (1990) describe the use of a flame to deposit films in what was termed a xe2x80x9cspray pyrolysisxe2x80x9d technique. Both Merkle et al. and McHale et al. deposited YBa2Cu3Ox from a combusted sprayed solution onto substrates, but the deposition conditions resulted in low quality pyrolysis and particulate type coatings. Koguchi et al. atomized a 0.03 mol/l aqueous solution and transported the resulting mist into a H2xe2x80x94O2 flame and deposited a 10 xcexcm thick coating in 10 minutes on a yttria stabilized zirconia (YSZ) substrate heated by the flame with much of the sprayed material being lost in transport due to the method used. The temperature, measured at the back of the substrate, reached a maximum of 940xc2x0 C. However, the flame side of the substrate is generally expected to be 100xc2x0 C. to 300xc2x0 C. higher in temperature than the back which would be in the melting range of YBa2Cu3Ox. The resulting Koguchi et al. film had a strong c-axis preferred orientation and, after a 850xc2x0 C. oxygen anneal for eight hours, the film showed zero resistivity at 90xc2x0 K. Koguchi et al. termed their method xe2x80x9cflame pyrolysis,xe2x80x9d and were probably depositing at temperatures near the melting point of YBa2Cu3Ox. The solution concentrations and deposition rates were higher than those useful in CCVD processing. Therefore, there exists a need for a coating method which achieves excellent results at below the coating materials"" melting point. The present invention fulfills this need because the finer atomization of the near supercritical fluid improves film quality by enabling the formation of vapor deposited films at lower deposition temperatures.
McHale et al. successfully produced 75 to 100 xcexcm thick films of YBa2Cu3Ox and Bi1.7Pb0.3Ca2Sr2Cu3O10 by combusting a sprayed solution of nitrates dissolved in liquid ammonia in a N2O gas stream, and by combusting nitrates dissolved in either ethanol or ethylene glycol in an oxygen gas stream. The results suggest the films were particulate and not phase pure. The YBa2Cu3Ox coatings had to be annealed at 940xc2x0 C. for 24 hours and the Bi1.7Pb0.3Ca2Sr2Cu3O10 coatings heat treated at 800xc2x0 C. for 10 hours and then at 860xc2x0 C. for 10 hours to yield the desired material. Even after oxygen annealing, zero resistivity could never be obtained at temperatures above 76xc2x0 K. The solution concentrations used were not reported, but the deposition rates were excessively high. In both Koguchi""s and McHale""s methods, the reported solution and resulting film stoichiometries were identical. Conversely, in CVD and in the present invention, the solution stoichiometry may differ from the desired film stoichiometry. Additionally the resulting droplet size of sprayed solutions was excessively large and the vapor pressure too low for effective vapor deposition.
A nebulized solution of precursors has been used with a plasma torch in a process termed xe2x80x9cspray inductively coupled plasmaxe2x80x9d (xe2x80x9cspray-ICPxe2x80x9d or xe2x80x9cICPxe2x80x9d). See M. Kagawa, M. Kikuchi, R. Ohno and T. Nagae, J. Amer. Ceram. Soc., 64, 1981, C7. In spray-ICP, a reactant containing solution is atomized into fine droplets of 1-2 mm in diameter which are then carried into an ICP chamber. This can be regarded as a plasma CVD process, different from flame pyrolysis. See M. Suzuki, M. Kagawa, Y. Syono, T. Hirai and K. Watanabe, J. Materials Sci., 26, 1991, p.5929-5932. Thin films of the oxides of Ce, La, Y, Pr, Nd, Sm, Cr, Ni, Ti, Zr, Laxe2x80x94Srxe2x80x94Cu, Srxe2x80x94Ti, Znxe2x80x94Cr, Laxe2x80x94Cr, and Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cu have successfully been deposited using this technique. See M. Suzuki, M. Kagawa, Y. Syono and T. Hirai, Thin films of Chromium Oxide Compounds Formed by the Spray-ICP Technique, J. Crystal Growth, 99, 1990, p.611-615 and M. Suzuki, M. Kagawa, Y. Syono and T. Hirai, Thin films of Rare-Earth (Y, La, Ce. Pr, Nd, Sm) Oxides Formed by the Spray-ICP Technique, J. Crystal Growth, 112, 1991, p.621-627. Holding the substrate at an appropriate distance from the plasma was crucial to synthesizing dense films. The range of desired deposition distances from the plasma source was small due to the rapid temperature drop of the gases. CVD type coatings were achieved using ultrasonically atomized 0.5-1.0 M solutions of metal-nitrates in water which were fed into the ICP at 6-20 ml/h using Ar flowing at 1.3-1.4 l/min. Only oxides were deposited and liquid or liquid-like solutions near the supercritical temperature were not used. The use of near supercritical atomization with ICP was not considered in this broad review of ICP nebulization techniques. See T. R. Smith and M. B. Denton, Evaluation of Current Nebulizers and Nebulizer Characterization Techniques, Appl. Spectroscopy, 44, 1990, p.21-4.
Therefore, it is highly desirable to be able to form nanopowders and coatings at low production and capitalization costs and with simple production processes. It is also desirable to be able to form coatings in the open atmosphere without any costly furnace, vacuum, or reaction chamber. It is further highly desirable to provide a coating process which provides for a high adhesion to the substrate, controlled microstructure, flexibility, reduced raw material consumption and reduced effect on the operating characteristics and/or dimensions of the coated system while being able to retain highly desirable submicron capabilities which are important to obtaining improved coating and powder formation. Moreover, it is highly desirable to provide a process which uses solutions near their supercritical point, and, therefore, achieves excellent results at below the coating materials"" melting point.
The present invention fulfills these needs and defines plasma torch and CCVD produced vapor formed films, powders and nanophase coatings from near supercritical liquids and supercritical fluids. Preferably, a liquid or liquid-like solution fluid containing chemical precursor(s) is formed. The solution fluid is regulated to near or above the critical pressure and is then heated to near the supercritical temperature just prior to being released through a restriction or nozzle which results in a gas entrained very finely atomized or vaporized solution fluid. The solution fluid vapor is combusted to form a flame or is entered into a flame or electric torch plasma, and the precursor(s) react to the desired phase in the flame or plasma or on the substrate surface. Due to the high temperature of the plasma much of the precursor will react prior to the substrate surface. A substrate is positioned near or in the flame or electric plasma, and a coating is deposited. Alternatively, the material formed can be collected as a nanophase powder.
The method of the present invention provides for very fine atomization, nebulization, vaporization or gasification by using solution fluids near or above the critical pressure and near the critical temperature. The dissolved chemical precursor(s) need not have high vapor pressure, but high vapor pressure precursors can work well or better than lower vapor pressure precursors. By heating the solution fluid just prior to or at the end of the nozzle or restriction tube (atomizing device), the available time for precursor chemical reaction or dissolution prior to atomization is minimized. This method can be used to deposit coatings from various metalorganics and inorganic precursors. The fluid solution solvent can be selected from any liquid or supercritical fluid in which the precursor(s) can form a solution. The liquid or fluid solvent by itself can consist of a mixture of different compounds.
A reduction in the supercritical temperature of the reagent containing fluid demonstrated superior coatings. Many of these fluids are not stable as liquids at STP, and must be combined in a pressure cylinder or at a low temperature. To ease the formation of a liquid or fluid solution which can only exist at pressures greater than ambient, the chemical precursor(s) are optionally first dissolved in primary solvent that is stable at ambient pressure. This solution is placed in a pressure capable container, and then the secondary (or main) liquid or fluid (into which the primary solution is miscible) is added. The main liquid or fluid has a lower supercritical temperature, and results in a lowering of the maximum temperature needed for the desired degree of nebulization. By forming a high concentration primary solution, much of the resultant lower concentration solution is composed of secondary and possible additional solution compounds. Generally, the higher the ratio of a given compound in a given solution, the more the solution properties behave like that compound. These additional liquids and fluids are chosen to aid in the very fine atomization, vaporization or gasification of the chemical precursor(s) containing solution. Choosing a final solution mixture with low supercritical temperature additionally minimizes the occurrence of chemical precursors reacting inside the atomization apparatus, as well as lowering or eliminating the need to heat the solution at the release area. In some instances the solution may be cooled prior to the release area so that solubility and fluid stability are maintained. One skilled in the art of supercritical fluid solutions could determine various possible solution mixtures without undue experimentation. Optionally, a pressure vessel with a glass window, or with optical fibers and a monitor, allows visual determination of miscibility and solute-solvent compatibility. Conversely, if in-line filters become clogged or precipitant is found remaining in the main container, an incompatibility under those conditions may have occurred.
The resulting powder size produced by the methods and apparatuses of the present invention can be decreased, and therefore, improved by: 1) decreasing the concentration of the initial solution; 2) decreasing the time in the hot gasses; 3) decreasing the size of the droplets formed; and/or 4) increasing the vapor pressure of the reagent used. Each of the variables has other considerations. For instance, economically, the concentration of the initial solution should be maximized to increase the formation rate, and lower vapor pressure reagents should be used to avoid the higher costs of many higher vapor pressure reagents. Decreasing the time in the hot gasses is countered by the required minimum time of formation of the desired phase. Decreasing the size of the droplets formed can entail increased fluid temperature which is countered by possible fluid reaction and dissolution effects. Similarly, coating formation has parallel effects and relationships.
Another advantage is that release of fluids near or above their supercritical point results in a rapid expansion forming a high speed gas-vapor stream. High velocity gas streams effectively reduce the gas diffusion boundary layer in front of the deposition surface which, in turn, improves film quality and deposition efficiency. When the stream velocities are above the flame velocity, a pilot light or other ignition means must be used to form a steady state flame. In some instances two or more pilots may be needed to ensure complete combustion. With the plasma torch, no pilot lights are needed, and high velocities can be easily achieved by following operational conditions known by one of ordinary skill in the art.
The solute containing fluid need not be the fuel for the combustion. Noncombustible fluids like water or CO2, or difficult to combust fluids like ammonia, can be used to dissolve the precursors or can serve as the secondary solution compound. These are then expanded into a flame or plasma torch which provides the environment for the precursors to react. The depositions can be performed above, below or at ambient pressure. Plasma torches work well at reduced pressures. Flames can be stable down to 10 torr, and operate well at high pressures. Cool flames of even less than 500xc2x0 C. can be formed at lower pressures. While both can operate in the open atmosphere, it can be advantageous to practice the methods of the invention in a reaction chamber under a controlled atmosphere to keep airborne impurities from being entrained into the resulting coating. Many electrical and optical coating applications require that no such impurities be present in the coating. These applications normally require thin films, but thicker films for thermal barrier, corrosion and wear applications can also be deposited.
Further bulk material can be grown, including single crystals, by extending the deposition time even further. The faster epitaxial deposition rates provided by higher deposition temperatures, due to higher diffusion rates, can be necessary for the deposition of single crystal thick films or bulk material.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.